This invention relates to a heavy-duty pneumatic radial tire for trucks, buses or the like, and more particularly to a pneumatic radial tire adapted to improve various tire performance, inter alia to effectively reduce the rolling resistance by properly modifying a radial surface profile of a carcass and simultaneously avoid deterioration of sidewalls due to aging by properly forming contours of the sidewalls.
It is known that in order to reduce the rolling resistance of a tire it is necessary to reduce the energy which is to be consumed due to periodic stresses and strains caused in the tire while rolling. It is a reasonable assumption for heavy duty radial carcass tires that rates of consumed energies at respective portions of tires are approximately 37% at treads, 23% at buttress portions, 20% at sidewalls and 14% at bead portions according to results of analysis on percentages of consumed energies of so-called radial carcass tires whose practical usefulness has been recognized under normal used conditions.
The tread has the largest contribution to the rolling resistance of the tire, and in order to reduce inner friction and hence the rolling resistance, therefore, the compounding ratio of tread rubber has been generally selected to increase its resilience. In this case, however, wet performance, which is one of the important performances of the tire, is adversely affected undesirably depending upon the degree of the reduced rolling resistance.
With the above solution, accordingly, the rolling resistance cannot be greatly decreased unless a particular precaution is taken to prevent the wet performance from being lowered. Since an effective method to maintain wet performance has not been found yet, the above solution does not achieve a significant effect.
It has been further proposed to use for sidewalls the rubber compounding ratio having reduced inner friction in the same manner as in the tread. This proposal, however, serves only to reduce 3% or less of the rolling resistance.
Moreover, it has been proposed that a ratio of area of grooves to total area of a pattern of tread of a tire be increased to restrain slight slippage in rolling of the tire so as to lower the rolling resistance. However, this method tends to lower a wear-resistance, so that it involves a limitation of the aimed effect.
The "proper modification of a radial surface profile of a carcass" means herein such a profile intentionally deviated from the naturally equilibrated configuration, which has been introduced from fundamental investigation as to deformation of sidewalls of tires in rolling and subjected to loads. This profile can be applied to tires having any flat coefficients.
In general, it has been known that the deformation of sidewalls is divided into bending deformation and shearing deformation. The invention resides in a discovery that radially inner portions of sidewalls (in the proximity of bead portions) are subjected to bending deformation much more than shearing deformation but the radially outer portions of the sidewalls (in the proximity of a tread) are subjected to shearing deformation much more than bending deformation.
The shearing deformation at the radially outer portions of the sidewalls is more complicated than the bending deformation. The inventors of the present application have carefully investigated these deformations to find the following important points.
First, during rolling under a load shearing deformation in the radially outer zones of the sidewalls of a tire is relatively small immediately below the load but considerably large in zones which are about to be in contact with and away from a ground surface, which takes a large part of the entire consumed energy.
Second, the shearing and bending deformations are in a reciprocal relationship where when one increases, the other decreases.
In view of the above fact, it has been found that even if the bending deformation concentrated immediately below the load in the radially outer portions of the sidewalls being comparatively thin increases somewhat, entire consumed energy can be reduced by decreasing the shearing deformation playing a large part in the consumed energy.
In a naturally equilibrated configuration, on the other hand, surfaces of radially inward zones of sidewalls likely to be deformed owing to bending are repeatedly subjected to radial compression strains caused by bending deformations in rolling. The directions of this compression are turned into directions perpendicular to the surfaces to produce shearing out of the surfaces because the surfaces have curvatures and are tilted to the directions of the compression.
As the result, the deterioration of rubber material due to action of ozone (O.sub.3) would be promoted in the radially inward zones of the sidewalls, so that cracks occur in the sidewalls in circumferential direction to lower durability of the case.
In the radial profile, which is intended to reduce the rolling resistance, bead portions greatly expand outwardly in filling inner pressure in the tire. In addition, although points on surfaces of sidewalls having prior art carcass lines move radially outwardly, they move radially inwardly in case of the radial profiles.
This radial inward movement causes large radial compressive strains on the surfaces of the tire, so that the deterioration of the sidewalls due to aging is promoted in conjunction with the above compression strains caused by rolling of the tire.
As a general solution for this problem, it has been only proposed to improve the rubber material or to coat the sidewalls with an aging resistant rubber material, which could not fundamentally solve this problem.